Paper has been used for a long time as a medium for display and storage of information. Notwithstanding the current trend of paperless information processing with the spread of flat panel displays, the use of paper is still increasing, far from decreasing. This seems to be because paper is a display medium of reflection type soft to the eye and is excellent in flexibility and handiness. Display media other than paper, for example, an OHP sheet comprising polyester, etc., on which a toner image can be formed, are also extending their use for obtaining a wide display.
However, to use large quantities of paper or plastics as a display medium has given a rise to an issue from the standpoint of resources saving or environmental conservation. Under such a situation, various display media which may substitute for paper have been developed. Examples of display media heretofore proposed include a reversible heat-sensitive display medium which comprises a high polymer matrix having dispersed therein an organic low-molecular weight compound and is capable of controlling light scattering and transmission through control of the heat applied as disclosed in JP-A-54-119377 and JP-A-55-154198 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"); a reversible heat-sensitive display medium comprising a film of a polymer blend which makes a display through heat-controlled phase separation as disclosed in JP-A-60-180887 and JP-A-62-116192; and a reversible display medium using a high polymeric liquid crystal which makes a display through a similar mechanism as disclosed in JP-A-2-117888 and JP-A-3-53285.
Recording on these reversible display media is carried out by application of heat under control usually by means of a thermal head or a thermal roll.
The reversible heat-sensitive display medium comprising a high polymer matrix film containing an organic low-molecular weight compound or the reversible heat-sensitive display medium comprising a polymer blend film reversibly switches between a light-transmitted state (transparent state) and a light-scattered state (opaque state) by controlling the compatibility of the binary system by means of heat. However, on repetition of the switching between these two states, the two components of the binary dispersed or mixed system are deteriorated and the distinction between the two states becomes unclear. It follows that the contrast is reduced and the number of times of repetition is so limited.
The known reversible display media using high polymeric liquid crystals have a low display contrast, insufficient record preservability, and insufficient heat sensitivity.
In addition, a recording layer comprising high polymeric liquid crystals tends to undergo deterioration due to heat for recording or surface deformation due to pressure applied on recording with a thermal head, also limiting the number of times of repetition. If a heat-resistant protective layer is provided on the recording layer for prevention of the surface deterioration, the high polymeric liquid crystal layer will be orientated due to shrinkage of the protective layer and become transparent.
It has been proposed to crosslink a high polymeric liquid crystal to improve heat resistance and thereby to prevent surface deformation on recording as disclosed in JP-A-2-42415. However, it is likely that crosslinking of a high polymeric liquid crystal interferes with orientation of the liquid crystal molecules, resulting in reduction of their characteristics.